Laser-based electrophotographic image-forming devices typically form images by scanning a laser across the surface of a photoconducting drum. Typically, a video signal causes a laser diode to pulse on and off as it's beam scans across the surface of the drum. Those portions of the drum surface that are struck by the laser beam undergo a physical change that enables the drum to pick up and then place toner on a sheet of paper.
Typically, the video signal is produced by an integrated circuit (IC) located on a raster image processor card. The video signal is transmitted over an electrical cable to an IC on an engine card. Usually, the video signal is then redriven over a cable from the engine card IC to a laser diode control card.
It is useful to think of printed images formed by electrophotographic image-forming devices as being constructed of printer elements, or pels. A pel is sometimes referred to as a dot, and the resolution of a printer is often characterized by the number of dots that it can print within a given linear distance, such as 600 dots per inch (DPI).
As shown in FIG. 1, a laser typically forms a dot by pulsing on and then off again for a given length of time within the pel. Often, a fifty percent duty cycle is used, such as illustrated in FIG. 1, meaning that the laser is off for approximately half of the total scan width of the pel, and on for approximately half of the total scan width of the pel. More specifically, the laser may be off as it scans across the first quarter of the pel, then on as it scans across the middle half of the pel, and then off again as it scans across the final quarter of the pel.
Among the general goals in the design of electrophotographic image-forming devices are increased speed and increased resolution. Increasing speed means decreasing the amount of time required to form an image on a page, or in other words, increasing the scan speed of the laser. Increasing the resolution of the image typically involves decreasing the size of the pels, or in other words, putting more pels within a given linear distance. Achieving either of these goals results in the laser operating at an increased frequency. For example, if the scan speed of the laser is increased, then it forms more pels per unit time. Since the video signal typically pulses the laser on and off for each pel as described above, the laser is pulsing on and off at a faster rate.
Similarly, if the resolution increases, then more pels are formed per unit scan distance of the laser. For example, if the 600 DPI resolution shown in FIG. 1 were increased to 1200 DPI, the width of each pel would decrease from 1/600 of an inch to 1/1200 of an inch. Assuming that the laser scan speed is not reduced, this increase in resolution also means that the laser is pulsed on and off at a faster rate.
The level of radiated electromagnetic interference (EMI) produced by a electrophotographic image-forming device tends to be related to the frequency at which the laser operates. Typically, increasing the operating frequency of the laser tends to increase the radiated EMI, and any reduction in the operating frequency tends to decrease the EMI. Radiated EMI from video data transmissions is also increased by each redrive of the video signal. The problem is magnified by the fact that often times the video signal travels from one card to another over long cables.
Governments regulate the amount of radiated EMI that a device such as a printer may emit. Thus, the level of radiated EMI produced by a printer is of great concern to printer manufacturers. In efforts to reduce radiated EMI, some manufacturers have reduced the length of the cable carrying the video signal, or reduced the voltage of the laser control signal to the laser, which tends to reduce the current. Other techniques include using coaxial cable or adding toroids to the cable. These methods tend to reduce the antenna effects of the cable, but they also tend to be expensive. Consequently, such methods are typically used as a last resort to save the product schedule.
Additionally, the quality of the video signal that drives the laser diode becomes more difficult to maintain as the video signal is driven over cables at high pel frequencies. The video signal quality can be significantly degraded by impedance mismatches between the conductors in the video cables and traces on the cards to which the conductors are connected. The effects of these impedance mismatches become more pronounced at higher video frequencies. As a result, the printed image quality tends to degrade as the frequency of the video signal increases.
What is needed, therefore, is an apparatus that reduces radiated EMI generated by a printer and maintains printed image quality, without significantly increasing the production cost or reducing the speed or apparent resolution of the printer.